Catalysts are basis of modern petrochemical industry, and supported metal catalysts, as an important class of catalysts, are widely used in applications such as oil refining, basic chemical feedstock preparation, fine chemicals industry, and the like. For example, Ni/SiO2—Al2O3 or Pd/molecular sieve catalysts have been used in hydrocracking to produce gasoline and other fuels; Pt/Al2O3 catalysts have been used in the catalytic reforming of naphtha to prepare high octane number gasoline, arenes and liquefied petroleum gas, and isomerization of light gasoline, alkanes or xylenes; Ni/Al2O3 catalysts have been used in methanation; Ag/Al2O3 catalysts have been used in the reaction for preparing ethylene oxide from ethylene; Pd/Al2O3 catalysts have been used in the selective hydrogenation of olefins, alkynes in pyrolysis gasoline or dienes, etc.
A supported metal catalyst consists typically of a carrier, a primary metal active component and an optional secondary metal active component. The carrier is a framework supporting the active components and also functions to enhance utilization rate of the active components, enhance heat stability of the catalyst, provide active centers, and the like. Commonly used carrier materials include alumina, silica, molecular sieves, active carbon, magnesia, titania, diatomite, and the like. The primary metal active component is generally a metal element with catalytic activity, and typically an element from Group VIII, such as Pd, Pt, Ni, and the like. The secondary metal active component may be used to modify the activity or selectivity of the catalyst, and commonly used secondary metal active components include Cu, Ag, Au, and the like.
Currently, a supported metal catalyst is typically produced by an impregnation-calcination process comprising contacting sufficiently a solution containing a metal active component precursor (typically, a solution of a salt) with a prepared carrier, to support the metal active component precursor on the carrier; and drying and then calcining at a high temperature the carrier having metal active component precursor supported thereon, to decompose the metal active component precursor into corresponding oxides. After loaded in a reactor, so-prepared catalyst is typically subjected to a pre-reduction treatment, i.e., reducing the metal oxides with hydrogen gas to elementary metal prior to use. Problems suffered by such impregnation-calcination processes for the preparation of a catalyst are that the calcination process consumes a large amount of energy, and that the high temperature involved in the process may cause the sintering of the metal active component particles and/or the carrier, resulting in the deterioration of the catalyst performance.
In order to avoid the influence of the sintering phenomenon on the catalyst performance, many of later catalyst preparation methods remove the high temperature calcination step, and use a chemical reduction process conducted at lower reaction temperature instead, along with heating or activating the system with ultrasonic wave, microwave, UV light, plasma, and the like, so that the catalyst performance is improved to some extent.
U.S. Pat. No. 5,968,860 discloses a method for preparing a hydrogenation catalyst useful in the gas phase production of vinyl acetate from ethylene, which method comprises supporting a Pd active component precursor and the like on a carrier and reducing the Pd active component precursor-supported carrier with sodium borohydride, hydrazine or formic acid at room temperature, wherein an ultrasonic wave activating step is included in the preparation. The resultant catalyst sample has a higher selectivity.
Chinese patent application CN 1579618 describes a method for preparing a supported metal catalyst, which method uses microwave radiation as heat source and a polyol as reducing agent and protecting agent, and can be used to rapidly prepare a multi-component supported catalyst having a supporting amount of from 1 wt % to 99 wt % and a particle size of metal particles controllable to 0.5 to 10 nm.
Chinese patent application CN 1511634 describes a method for preparing a catalyst useful in the selective hydrogenation of ethyne to ethylene. The method uses a radio-frequency plasma to activate and decompose a Pd precursor supported on Al2O3 at mild conditions and then carries out H2 reduction, to give a catalyst characterized by high low-temperature activity and high selectivity.
U.S. Pat. No. 6,268,522 utilizes UV light reduction process to prepare a hydrogenation catalyst. Irradiating a carrier having an active component precursor and a sensitizing agent impregnated thereon with UV light will cause reduction in the surface layer portion so that the active component will be distributed in an eggshell shape, and the shell thickness can be controlled via the conditions, e.g. UV radiation wavelength, radiation power and irradiating time. After extracting the un-reduced active component precursor with a solvent, the resultant sample exhibits good activity and selectivity in the reaction for the gas phase production of vinyl acetate from ethylene.
In the above improved methods, microwave and UV light belong to electromagnetic radiation. Microwave is an electromagnetic wave having a wavelength ranging from 1 to 1000 mm, and it heats a system through rapid turn of polar molecules under the action of high frequency electric field and is a heating method per se. UV light is an electromagnetic wave having a wavelength ranging from 10 to 400 nm, and its photons have an energy range in accordance with that required to excite molecules so that they can be selectively absorbed by the molecules to excite the molecules and cause chemical reactions. Ultrasonic wave is mechanical vibration, and it applies some influence on the performance of a catalyst through the action of vibration energy on the catalyst. Plasma belongs to low-energy, charge-born particles, and it decomposes and activates an active component precursor through complex chemical reactions between the large amount of charge-born particles and the active component precursor. In the above improved methods, the plasma treatment is an activating method replacing for the calcination step, microwave and ultrasonic wave essentially provide heat source to the chemical reduction process, and only UV radiation can cause reduction reaction of the active component precursor. However, since UV light has a poor penetrating ability for a solid, it can act on only the surface layer of the catalyst and can hardly be used in the production of a mass of product. Furthermore, these methods involve complicated operations, and generally require the use of a large amount of compounds as reducing agent, protecting agent or solvent. Taking into account the economic issues involved in the preparation of a mass of a catalyst, it is difficult for these methods to be used in commercial production.
Thus, there is still a need for providing a simple, effective method that can be used to prepare a supported metal catalyst with good activity and selectivity.